The importance of quantifying molecular/cellular events cannot be overemphasized for molecular imaging. For example, the use of nanoparticles as delivery vehicles for diagnostic and therapeutic agents requires accurate counts of nanoparticles accumulated at the diseased tissue to make diagnostic decisions and gauge therapeutic dose. The measurement of drug dose at targeted sites is essential for monitoring therapy. Quantitative accuracy and reproducibility have to be established to standardize and cross-validate molecular MRI methods. So far there is a lack of effective tools to quantify molecular/cellular events. In this work we will use quantitative susceptibility mapping (QSM) to map the distribution of super-paramagnetic iron oxide (SPIO) particles loaded with Paclitaxel. The successful execution of this work will lead to a robust, quantitative and accurate method for mapping magnetic markers in MRI. The resulting technique can be used to map and measure local drug dose delivered by magnetic nanoparticle vehicles targeting diseased tissue cells